Techniques for rendering volumetric data sets are known in the art. These techniques have been described in detail. See, for example, Marc Levoy, A Hybrid Ray Tracer for Rendering Polygon and Volume Data, IEEE Computer Graphics and Applications, pages 33-40, March 1990; and Marc Levoy, Efficient Ray Tracing of Volume Data, ACM Transactions on Graphics, 9(3):245-261, July 1990, both of which are incorporated herein by reference.
Fast rendering of volumetric data sets using the texturing capabilities of graphics hardware has also been described in detail. See, for example, B. Cabral, N. Cam and J. Foran, “Accelerated Volume Rendering and Tomographic Reconstruction Using Texture Mapping Hardware”, ACM Symposium on Volume Visualization, 1994; C. Rezk-Salama, K. Engel, M. Bauer, G. Greiner, T. Ertl, “Interactive Volume Rendering on Standard PC Graphics Hardware Using Multi-Textures and Multi-Stage Rasterization”, In Euographics/SIGGRAPH Workshop on Graphics Hardware, 2000, pages 109-118, 147, Addison-Wesley Publishing Company, Inc., 2000, which are incorporated by reference. Shading models for volumetric data are also discussed in N. Max, “Optical Models for Direct Volume Rendering”, IEEE Transactions on Visualization and Computer Graphics 1995, pages 99-108, which is incorporated herein by reference.
Shading is a well-known technique to enhance depth-perception and accentuate details contained within the data that can be performed in real-time. Shading volumetric data requires gradient information for each voxel. Gradients can either be pre-computed and stored along with the volume data or computed on-the-fly. Pre-computing gradients significantly increases the amount of memory required, thereby making this approach impractical for large volume data sets. Clinical routines currently generate data sets exceeding half a gigabyte, and the amount of information being acquired is increasing rapidly. New systems are capable of generating data sets that are more than a gigabyte in size. Furthermore, gradients are typically quantized to either 8 or 16-bits to constrain the amount of memory required, which decreases the quality of the volumetric renderings. On the other hand, although computing gradients on-the-fly provides better quality, and neither requires pre-processing nor imposes any significant additional memory demands, it suffers from significantly slower performance. This reduction in performance is due to the additional computations and memory accesses during rendering; for example, a forward difference gradient requires accessing three neighboring voxels, while a central difference gradient requires accessing six neighboring voxels.
It is therefore desirable to provide methods and systems that perform faster on-the-fly gradient estimation for shaded rendering of volumetric data.